1. Field of the Invention
The present invention relates generally to vapor deposition methods and, in particular, a plasma sputtering apparatus by which thin or thick films of insulating, semiconductor, or conductive materials are formed. Potential applications exist in the fabrication of integrated circuits, optical elements, optoelectronic devices, and other such products requiring well-controlled physical properties in these same films.
2. Description of the Related Art
In one aspect, the invention relates generally to the treatment of dispersed photo absorbing media, such as gases, with ultraviolet radiation. An equipment geometry for this purpose, in fluid treatment, utilizes a flow-through geometry, wherein the media to be processed passes through a processing tube constructed of ultraviolet-transmitting material--such as fused silica--and wherein the tube is surrounded with one or several ultraviolet lamps, thereby creating a high radiative flux within the photo absorbing media. The coupling efficiency of the ultraviolet radiation to the media may then be increased, by placing this coaxial arrangement within a reflective cavity. This latter reflective cavity becomes increasingly necessary as the extinction distance of the ultraviolet within the media becomes much greater than the relevant physical dimension of the apparatus, and the ultraviolet radiation must make many passes through the media before it is appreciably absorbed.
The problems encountered with irradiating low absorption cross-section dispersed media become increasingly acute with lower pressure processes, wherein the dispersed media would typically be some gas or vapor which is rarified to a degree consistent with the level of vacuum. In these latter vacuum processes, one encounters situations wherein the absorbing constituent may have a vapor pressure of only 10.sup.-6 atmospheres, with extinction distances in the range of 10.sup.2 to 10.sup.3 meters. At the same time, these vacuum processes will frequently involve one or several critical material surfaces that interact with the process quite differently when irradiated with the ultraviolet radiation. These same critical surfaces will typically be modified during the process, so that the result of irradiating these surfaces will change, as well. For instance, a thin film forming on one of these material surfaces can dramatically alter the absorption, scatter, or reflection of the ultraviolet radiation as its thickness increases. In ultraviolet-enhanced physical vapor deposition (PVD) processes, including reactive processes utilizing PVD sources, these issues have not been adequately addressed.
A prevalent PVD means in industry for the deposition of high quality thin films is through the utilization of sputtering techniques. The term "sputtering" refers to a group of mechanisms by which material is ejected from a solid, or sometimes a liquid, target surface into a vapor form; this latter effect being due, at least in part, in either physical or chemical sputtering, to the kinetic energy transferred to the target atoms or molecules by bombarding particles. These mechanisms are utilized in sputter deposition processes categorized generally as laser sputtering, ion beam sputtering, glow discharge (or diode) sputtering, and magnetron glow discharge sputtering. The present invention, in its preferred embodiment, concerns primarily plasma sputtering, and, in particular, magnetron plasma sputtering. The magnetic confinement of the sputtering plasma in the magnetron sputtering process allows for a far greater range of mean free paths than the earlier, capacitively coupled diode plasma sputtering process. Its high deposition rate, combined with its versatility in depositing a wide range of materials under a great range of conditions, has made magnetron plasma sputtering a preferred thin film deposition technique for many industrial applications.
Yet, there are several aspects of plasma sputtering which are seen as significant barriers in utilizing the technique for future industrial applications. Most commercially available plasma sputter sources provide a small proportion of ionized species to the depositing film (&lt;5%). Most of the energy supplied for non-equilibrium growth is supplied by the thermal velocities of the depositing species. The thermal distribution of these velocities is necessarily broad, allowing little control over specific growth processes at the film growth interface. Because the energy supplied by the depositing species is kinetic, it is often difficult to provide high energies to the growth interface with out simultaneously causing subsurface damage, due to the recoil and implantation of the bombarding atoms.
Several modifications have been devised to render greater control over plasma sputtering processes wherein, as in the present invention, excited state and ion populations in the gas/vapor phase are increased and manipulated by means external to the sputtering plasma. This is most commonly accomplished by injecting electrons into the sputtering plasma to increase the plasma density and ion population, while simultaneously allowing a decrease of the target voltage. A resulting benefit is the ability to introduce a high proportion of relatively low energy ions to either etch or deposit on the substrate. This method has been made popular in the well-established triode and tetrode sputtering configurations, wherein electrons are usually supplied by a thermionic filament. This latter art has been found to work well for the deposition of metals, but is not compatible with reactive processes where electron emitting surfaces are prone to modification.
In recent years, plasma sputtering processes have also been developed that increase ionization through the utilization of secondary coils or antennas for RF or microwave excitation of the plasma. This latter prior art has also been found useful in the deposition of metals. However, difficulties arise, in that resonance conditions are effected by the inevitable modification of the process chamber surfaces during deposition, especially when depositing insulating or semi-insulating materials; also, these latter developments offer little resolution of the plasma species to be ionized.
The use of sources of UV/optical energy in conjunction with sputtering plasmas is relatively limited compared to the prior art concerning electron sources. In various instances, plasma sputtering experiments have been conducted utilizing the geometry set forth in U.S. Pat. No. 4,664,769 issued May 12, 1987 by Cuomo et al. This patent teaches a method wherein a UV source is directed onto a sputtering target during the magnetron sputtering process. The UV wavelength used is of an energy of or exceeding the photoelectric threshold of the target material, thereby causing the target to emit photoelectrons into the sputtering plasma. This photoelectric addition of electrons is found to increase plasma density, lower the cathode voltage required to sustain a discharge, as well as to increase the ion flux to the substrate, enabling modification of the film properties. As this work focuses on the irradiation of the sputtering target, its operation is contradictory to the goals of the present invention.
The use of UV/optical sources with magnetron sputtering plasmas in later work has consisted of efforts wherein a UV source, usually a laser, is directed upon the substrate being processed. These experiments are conducted in order to promote and study various surface reactions and solid phase transformations at the substrate surface, sometimes with a reactive gas injected at the substrate. As such, these accounts deal with UV interactions with the substrate surface and do not anticipate the present invention.
The use of UV/optical radiation sources in combination with processing plasmas has consisted mostly of the research conducted in relatively higher pressure photo-enhanced and plasma-enhanced chemical vapor deposition (CVD) processes. In the relevant accounts, ultraviolet radiation, usually from a laser, irradiates the substrate upon which the thin film is being deposited. This work in CVD was originated by Hargis, Gee, et al, and reported in the publications, "Laser-plasma interactions for the deposition and etching of thin-film materials", wherein is described the mechanism by which laser-produced UV activates the top monolayers which are plasma-deposited on the substrate.
This initial work utilizing both plasmas and UV radiation sources in CVD has continued. Researchers have since found that the plasmas used for plasma-enhanced CVD and plasma-enhanced chemical etching may be simultaneously or separately used as a photochemical UV source. The interaction of UV with these plasma-enhanced CVD and chemical etch processes has been found to take place primarily in surface modifications, such as in photo-activation of heterogeneous surface reactions at the substrate surface; because of this, these process geometries must incorporate means for illuminating the substrate surface which is being modified. Any photo-activated gas-phase reactions which might, in addition to the surface interactions, occur in these CVD and chemical etching plasmas would be essentially non-existent in the low-pressure, higher power density sputtering plasmas; nor are such gas phase reactions a necessary element of the present invention.
While the use of UV sources is a promising route for enhancing and controlling film growth and etching processes in plasma processing, the aforementioned prior art has had little impact on sputtering deposition/etching applications. Reasons for this are viewed, in the present invention, in light of the highly non-equilibrium thermodynamic mechanisms inherent in plasma sputtering technology. While the plasma sputter source provides a reliable means for depositing many materials under a wide range of conditions, consistently achieving a specific resultant film structure and composition, within relatively tight tolerances, remains a formidable challenge. Introducing additional energy sources to the sputtering plasma further complicates issues of stability and repeatability.
The prior art invariably utilizes process-altered surfaces which receive UV energy, namely the sputtering target or the substrate; but, in addition, chamber walls and fixturing. Any solid surfaces which might potentially receive UV radiation must act as a transmitting, reflecting, absorbing, or scattering surface. Because these process surfaces tend to be altered during the deposition process, the interaction of the UV source with the deposition process is also altered. As the reflectivity, scattering, and absorption occurring at these surfaces changes with process time, plasma-related mechanisms occurring throughout the process volume, such as secondary electron emission, gas/vapor photo-excitation, radiant heating, and photon-assisted sputtering, can all be dramatically altered. Hence, UV radiation incident on a growing film, or on the sputtering target, can interact with the deposition process in an unstable fashion.
The terms "plasma" and "discharge" both refer herein to the general sense of an electrically or electromagnetically sustained, photo-emitting, gas/vapor discharge, wherein quasineutrality of the gas/vapor may not necessarily exist. While the term "plasma" has been used more restrictively, and certainly more inclusively, than in the definition offered herein, the latter definition is consistent with current-day usage in the semiconductor industry, vapor deposition sciences, and other areas where the present invention might find application. The two terms are utilized differently in the present disclosure as a means of clearly differentiating between the sputtering "plasma" of the preferred embodiment, and the photoemitting "discharge" of the disclosed ultraviolet lamp source.
In disclosing the present invention, the terms "cavity", "reflective cavity", and "optical cavity", will all refer to the common and general sense of a predetermined structure for confining propagation of optical radiation between reflective surfaces.